1. Field of the Invention
The invention relates to a contact point constitution that makes and breaks a direct current load and a switching mechanism such as a relay and a switching mechanism having the contact point constitution.
2. Description of the Related Art
As a contact point material for a relay or a switching mechanism that makes and breaks an electric circuit, from viewpoint of performance and price, AgCdO alloy has been generally used. When the material is used in movable contacts and stationary contacts, under either direct current loads of direct current resistance load and direct current induction load, problems such as the conduction defect due to consumption of the contact point, locking due to material transfer from one contact point to other contact point, welding between contact points, and abnormal arc continuation have not been experienced over a long period of time. However, since the AgCdO contact point contains a hazardous material, Cd, in recent years, a movement against the use of the relays and switches that use cadmium is gathering strength. In such movement, development of switching mechanisms that use contact point materials capable of substituting the AgCdO contact points is urgent.
Technology that uses, as the contact point material that does not contain cadmium (hereinafter referred to as “cadmium-free contact point materials”), silver-tin oxide-indium oxide system contact points (hereinafter referred to as “AgSnO2In2O3 system contact point”), silver-tin oxide system contact points (hereinafter referred to as “AgSnO2 system contact point”), silver-nickel system contact points (hereinafter referred to as “AgNi system contact point”), silver-zinc oxide system contact points (hereinafter referred to as “AgZnO system contact point”) and soon has been developed. In such technology, the above contact point materials each can be independently used as a contact point material common to both of the movable contact point and stationary contact point. However, since, in such technology, there are strong and weak load regions of load-breaking switching mechanisms, the above contact point materials cannot necessarily substitute for the AgCdO contact points in both direct current loads of direct current resistance load and direct current induction load. For details, when the above contact point materials each are independently used as the contact point material common to the movable contact point and stationary contact point, under the direct current induction load, problems such as {circle around (1)} conduction defect due to consumption of the contact point, {circle around (2)} locking due to material transfer from one contact point to other contact point, {circle around (3)} welding between the contact points, and {circle around (4)} abnormal arc continuation are caused. Furthermore, under the direct current resistance load, the problems such as above {circle around (2)} through {circle around (4)} are caused. Thus, it is very difficult to replace, by independently using the above cadmium-free contact point materials each as the common contact point material, the AgCdO contact point under both load conditions.
In particular, among the above-mentioned cadmium-free contact point materials, the AgZnO system contact points, though used only in some cases in breakers and so on that are relatively small in the number of makings and breakings, from the following reasons, are seldom used in the switching mechanisms such as relays that are frequently made and broken.
(1) Since the AgZnO system contact point is low in the consumption-resistance, there is danger of insulation deterioration.
(2) Since the AgZnO system contact point is low in the consumption-resistance, the number of lifetime is short.
(3) Since the AgZnO system contact point is very high in the hardness, it is difficult to process into a small contact point.
The AgSnO2InO3 contact point is much in the transfer of the contact point when the direct current induction load is made and broken and frequently causes a problem in that the abnormal arc continuation results. Accordingly, the AgSnO2InO3 contact point can be applied to the direct current induction load with difficulty.
In order to enable the cadmium-free contact point material to substitute for the AgCdO contact point in both direct current load conditions of the direct current resistance load and the direct current induction load, a structure of the switching mechanism is tried to largely revise. However, there is a problem in that the large revising takes a very long period of time and needs much expense.
Furthermore, although different cadmium-free materials are tried to use separately as the movable contact point material and the stationary contact point material, it is also difficult to always replace the AgCdO contact point in both of the direct current resistance load and the direct current induction load. That is, under the both of the above loads, the problems from {circle around (1)} to {circle around (4)} are not always overcome.
Accordingly, it is considered to prepare in advance a switching mechanism that can inhibit the above problems from occurring only under the direct current resistance load that has no inductivity and a switching mechanism that can inhibit the above problems from occurring only under the direct current induction load that has inductivity and to use these according to the inductivity of the loads. However, the selection of the contact point material has to be decided depending not on the inductivity of the load thereto the switching mechanism is applied but on a magnitude of the inductivity of the load (in general, time constant and magnitude of inductance). That is, in the direct current inductance load, the magnitude of the inductivity of the load is various depending on the kind of the load. Accordingly, when the switching mechanism that does not cause the above problems under the direct current induction load that has particular inductivity, because of being suitable to the direct current induction load, is applied to the direct current inductance load that has the inductivity different from the above inductivity, the problems cannot be necessarily inhibited from occurring. Accordingly, actually the selection of the contact point material has to be carried out while confirming the magnitude of the inductivity of the load to be applied, that is, it is remarkably troublesome.
The invention is carried out in view of the above circumstances and intends to provide a direct current load breaking contact point constitution that can make and break an electric circuit over a long period of time under both direct current loads of the direct current inductance load and the direct current resistance load without causing problems such as {circle around (1)} the conduction defect due to the consumption of the contact point, {circle around (2)} the locking due to the material transfer from one contact point to other contact point, {circle around (3)} welding between the contact points, and {circle around (4)} the abnormal arc continuation; and a switching mechanism having the above constitution.
In the specification, {circle around (1)} “the conduction defect due to the consumption of the contact point” means a phenomenon in which because of the consumption of the contact point, a movable contact point and a stationary contact point do not come into contact or a phenomenon in which although the movable contact point and the stationary contact point are in contact but are not in conduction. It is considered that when the contact points are separated under the direct current induction load, since a relatively large energy stored in the load (arc discharge energy) is discharged at once, the contact point material causes not only the transfer described later in {circle around (2)} but also the sticking to the surroundings of the contact point, resulting in consuming one contact point (negative electrode side) and causing the conduction defect. In the direct current resistance load, such energetic arc discharge as in the direct current induction load is not caused that such conduction defect is not caused.
{circle around (2)} “The locking due to the material transfer from one contact point (negative electrode side) to the other contact point (positive electrode side)” means a phenomenon in which concavities and convexities that are generated owing to the transfer of the contact point material between surfaces of different contact points lock each other and the movable contact point and the stationary contact cannot be separated or are delayed in the separation. Such phenomenon can be caused in both of the direct current induction load and resistance load. However, in the direct current inductance load, the material transfer is caused substantially only in one direction from the negative electrode side to the positive electrode side, and in the direct current resistance load, the transfer can be caused in both directions of from the negative electrode side to the positive electrode side and vice versa.
{circle around (3)} “The welding between the contact points” means a phenomenon in which because of the melting of a surface of the contact point, the movable contact point and the stationary contact point stick each other and cannot be separated or are delayed in the separation. The phenomenon can be caused in both direct current loads of the direct current resistance load and the direct current induction load.
{circle around (4)} “The abnormal arc continuation” means a phenomenon in which despite of complete separation of the movable contact point and the stationary contact point, the arc discharge continues for a relatively long period of time (for instance, several hundreds milliseconds or more) between the movable contact point and the stationary contact point. The phenomenon can be caused in both direct current loads of the direct current resistance load and the direct current induction load.